1. Field of the Invention
The present invention relates to multiple file servers or a network attached storage (NAS), and to file system migration in the multiple file servers or the network attached storage (NAS).
2. Description of the Related Art
Hitherto, a network attached storage (NAS) having a storage system connected to a network and used as a storage disk shared with a network-connected computer has been proposed. The NAS is composed of a server including a network interface, and a disk drive for storing data.
For example, U.S. Pat. No. 6,671,773 discloses a cluster type NAS having a plurality of servers each connected to a network. In the system disclosed in this publication, a network element, a switching element, and a disk element correspond to a server of the NAS. It is possible to provide a plurality of network elements that share a file system. Further, a plurality of disk elements enable disk-based migration. Further, the respective network elements can access all file systems of the disk elements. Even when a disk storing any file system migrates between the disk elements, the individual network elements can access all the file systems independently of the migration.
As an example of the file system for accessing files distributed on the network, a network file system (NFS) has been proposed. According to NFSv4 (RFC3530), which is the latest version (see, RFC3530, NFS version 4, pp. 58-61, on the IETF home page, when a file system migrates from one server to another, the server notifies a client of information about a location of a migration destination in response to an access to the file system from the client, and the client can access the file system at the migration destination in accordance with the notified information.
In the case of migrating a file system (or a part of the file system) in the cluster type NAS with servers, it is necessary to copy file data between disk devices in the individual servers of the NAS. This causes problems that the migration takes much time to execute, and a client's accessibility to the file is lowered due to file data copying for the migration.
For the purpose of making the data copying unnecessary during the migration, there has been proposed a method of sharing all the devices among all servers and changing the server to access a target device in response to the migration. In this case, however, all the servers need to recognize all the devices in the system. The number of devices recognized by one server is limited under the constraints on resources of the server. Owing to this upper limit, the number of devices in the system is limited.